Gas-liquid contact towers are so widely used e.g., distilation, absorption, gas scrubbing, that they have become subject matter for a considerable number of practical and theoretical studies over the years. Large bodies of art exist to each aspect of tower structure and operation, and many efficient tower designs are available to one skilled in the art. Whole classes of tower types exist, e.g., packed towers, tray towers. In addition each class of towers is offered with numerous gas-liquid contact elements, e.g. target or impingement trays, sieve trays, valve trays. This invention is adapted principally to the class of gas-liquid contact towers which can be identified as tray towers and which briefly comprise vertical towers built with horizontal spaced apart trays. The liquid cascades from down tray to tray as the gas flows up through the trays, e.g. through bubble caps or perforations.
Although highly efficient gas-liquid towers are available to the art, their high efficiency depends very much upon existence of essentially constant steady state conditions with little variation in flow rates for gas and liquid. The tower operates efficiently at design capacity, relatively well at just above or below design capacity but woefully inefficiently when gas flow and/or liquid flow is far from the design flow rate e.g. at 25% of design. Difficulties caused by high turn down from design capacity are particularly severe in the instance of gas scrubbers, because high turn down is common and equipment often must be operated at partial capacity (certainly more often than in distillation columns, for example).
The art has, of course, appreciated existence of the turn down problem, and also that this problem can not be resolved to any substantial degree by more efficient tray or packing structures. Some of the approaches to this problem heretofore suggested to the art have been to:
1. Recycle the gas PA1 2. Bleed in outside air PA1 3. Employ gas dampers that block off part of the tower PA1 4. Employ a number of small towers in lieu of a single larger tower.
The recycle and bleed in approaches (1) and (2) consume energy. Power costs increase at low gas flow rates. In addition, dilution of the incoming gas, by recycle or outside air is often detrimental to the gas-liquid contact process. In the instance of the recycle approach the necessary piping and duct connections add significantly to the cost of the gas-liquid contact tower.
The big dampers capable of blocking off part of a large tower (for approach (3) are unwieldy, expensive and unreliable.
Their higher cost is the big objection to employment (for approach (4) of a multiplicity of smaller towers in lieu of a single large tower.
In total, the art still needs a sophisticated low construction cost approach toward improving tower efficiency at high turn down from design capacity flow rates.
It is an object of this invention to provide a gas-liquid contact tower capable of efficient operation at high turn down from design capacity.
Other objects and the advantages of the present invention will be apparent from the description of this invention which follows.